Population expert: The 11-billion-person planet

This year will see the world’s 7-billionth person. If only population growth forecasts were as definite, says demographer John Bongaarts

By how much do you think the world’s population will grow this century?The UN predicts that we will reach 9.1 billion in 2050, then the peak will occur around 2070 at 9.4 billion. But it is so far ahead and the margin of uncertainty is huge. We could easily end up with 1 or 2 billion more, or less.

What factors could lead to 11 billion people?The UN assumes fertility in Africa, for example, will …